Elastomeric copolymers of vinylidene fluoride having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses.
Production of such fluoroelastomers by emulsion polymerization methods is well known in the art; see for example U.S. Pat. Nos. 4,214,060 and 4,281,092. The result of the polymerization is a dispersion or latex of the polymer. Generally, fluoroelastomers are then separated from the dispersion by addition of an coagulant to form a slurry. The slurry is then washed and dried and then shaped into final form for commercial use.
Coagulants heretofore employed are typically salts of inorganic multivalent cations, A. L. Logothetis, Prog. Polym. Sci. 14, 251-296 (1989). These include aluminum salts such as aluminum sulfate, alums such as potassium aluminum sulfate, calcium salts such as calcium chloride and calcium nitrate, and magnesium salts such as magnesium chloride and magnesium nitrate. While these salts work very well as coagulants, residual amounts of these salts remain in the polymer. The presence of these salts renders these polymers unsuitable for use in contamination-sensitive applications such as seals in semiconductor manufacture. Thus, it would be desirable to find other coagulants effective for use in the emulsion polymerization of fluoroelastomers.
Salts of univalent cations, such as sodium chloride, have been proposed as coagulating agents for the manufacture of fluoroelastomers. Residual amounts of these salts are considered relatively innocuous in some end use applications. However, excessively large amounts of salts of univalent cations are required to fully coagulate the fluoroelastomer. The resulting polymer is difficult to fully dry In addition, the large amounts of these salts that are needed to coagulate the polymer require large and expensive water treatment facilities.
The use of organic coagulants is another method to avoid polymer contamination. Residual amounts of organic coagulants will not contaminate semiconductor processes and in any case may volatilize out of the polymer during the curing process. U.S. Pat. No. 3,598,794 discloses polyamines as coagulants for fluoroelastomers. Addition of a polyamine to a fluoroelastomer dispersion forms a gel that can be separated from the aqueous phase. Washing of this gel, however, is difficult and residual polyamine that remains in the fluoroelastomer interferes in the curing operation.
U.S. Pat. No. 3,997,705 discloses coagulation of a fluoroelastomer with an organic base or salt that acts as a vulcanization accelerator. However, the use of such a coagulant results in a fluoroelastomer that is subject to premature cure or scorch. In addition, the use of such a coagulant restricts the options for subsequent compounding of the fluoroelastomer, since an accelerator is already present in the polymer as received by the customer.
Perfluoroelastomers have been coagulated with organo onium compounds (US 2005/0143523). However, the presence of residual organo onium compounds in the resulting elastomer gum can cause premature vulcanization (i.e. scorch), making processing difficult.
Copending U.S. Patent Application 60/900,264 (filed Feb. 8, 2007) discloses fluoroelastomer coagulants that are water-soluble polymers having at least two quaternary onium centers per polymer chain.